Drag Measurement

Glenn
Research
Center

Aerodynamicists use
wind tunnels
to test
models
of proposed aircraft and engine components.
During a test, the model is placed in the
test section
of the tunnel and air is made to flow past the model.
Various types
of instrumentation are used to determine the forces on the model.
The most basic type of instrument is the force balance.

Force balances are used to directly measure the aerodynamic
forces and
moments on the model.
On this page, we will discuss the measurement of the
drag force
to demonstrate the basic principles involved with force balances.
This configuration is called a one component balance since it
only measures one force.
There are more sophisticated,
three-component balances that can
simultaneously measure
lift.
drag and
pitching moment.
A six-component balance is required to
measure all three forces (lift, drag, and side) and three moments
(pitch,
roll, and
yaw)
that determine an aircraft's motion through the air.

In the figure, we see a picture of the Wright Memorial Tunnel (WMT) at the NASA Glenn Research
Center. The WMT is used for student projects and as a travelling exhibit to let the public
see how wind tunnels operate. The top speed in the test section is about 50 mph.
Here is a more detailed
picture showing a
Joukowsky
wing model mounted in the
test section
of the tunnel.

The model is mounted on a
sting
attached to the wing's trailing edge.
The left part of the sting passes through
the bottom wall of the tunnel and attaches to a
strain gage
on the outside of the tunnel. The strain gage is contained in the silver box located under the
test section of the wind tunnel. Output from the strain gage is fed to a transducer that
converts the measurements into drag data that is displayed on a laptop computer.
During testing, the
angle of attack
of the wing is varied to produce a performance curve for the model.
A variety of models with different values of
camber and thickness were tested in the tunnel. These models can
be seen in the shoe box at the bottom of the upper figure.
After wind tunnel testing, the data was curve fit and is now made available in the
FoilSim III computer program.

This method of determining drag requires an electronic strain gage and a laptop computer
to convert the electronic output into units of force.
To accurately determine the aerodynamic forces and moments on an aircraft model in a "real"
wind tunnel requires even more sophisticated instrumentation and larger computer systems
for data reduction and display. Multiple electronic
strain gages
are often placed
inside
the model, or on a measuring
platform
outside the tunnel.
Multiple gages permit the determination of multiple forces and moments
during the same test.

Some
student-built
wind tunnels are equipped with much simpler
mechanical methods
to measure lift and drag. In place of the electronic strain gage,
a simple
spring scale
can be used if the forces are small.
In either case, the instruments must be calibrated against a known
amount of force before and during a wind tunnel test.

The wind tunnel data used in the FoilSim III computer program was
obtained by Anthony Vila, a student at Vanderbilt University, during a
summer intern session at NASA Glenn in 2010.